1. Technical Field
This invention relates to personal computer systems and in particular to a method and system for improving operation of direct access data storage devices (DASDs) used with such systems. Still more particularly the invention relates to selectively hiding marginally defective areas of a disk in a disk drive from an initiator during formatting of the disk and during subsequent inquiries by an initiator about the number and location of these areas.
2. Description of the Related Art
Disk drives are one type of DASD used for long term storage of data in personal, and other, computer systems. A disk drive has at least one rotatable magnetic disk for data storage. Data are physically recorded as a series a magnetically polarized regions on the disk. The magnetically polarized regions are disposed in a layer of magnetic material coating the major surfaces of the disk. Related magnetically polarized regions are serially arrayed along either a plurality of concentric data tracks or spiral data tracks. A read/write transducer may be used to read data from or write data to the various tracks on the disk while the disk moves underneath the transducer.
While it is desirable to make the layer of magnetic material or storage media free of defects, the presence of areas which are irregular is not cause for rejection of the disk provided that the areas are not too extensive. That is, the irregular areas cannot be too irregular or cover so much of the disk that the disk cannot meet a guaranteed minimum storage capacity. However, just what in the nature of an irregularity of the magnetic layer constitutes a defect depends on a number of factors, chief among which is areal density of the data to be recorded. For example, in disk drives using "sliders", the slider carries the read/write transducer and flies just off the surface of a rotating disk. Progress in reducing slider fly height has made the criteria for judging a magnetic layer "defect free" ever more stringent. As the read/write transducer comes closer to the disk surface, higher areal densities of data are achieved. Put another way, the apparent size of flaws becomes larger and thus smaller flaws constitute defects. A high degree of surface smoothness to assure close control of fly height and great regularity in quality of the material, are required to support higher areal densities.
A factor which could support a looser definition of "defect" is the use of error correcting codes (ECC) in association with data fields being physically recorded. ECC is a type of redundant data. A detailed understanding of how ECC works is not important to understanding the present invention. What the use of ECC allows is the tolerance of some level of error (either soft or hard) in the data field without loss of the data in the field. If all of the physical records on a disk were backed by redundant data such as ECC, the disk might have the same number and type of irregularities as a disk not receiving data not backed by ECC, but would be taken to have fewer defects. Incidentally, soft error is the incorrect readback of a correctly recorded datum. A hard error is an error in the record.
However, various parts of a disk surface will typically be used for storing particular types of data, some of which may be backed up by use of ECC and some of which is not. For example, in hard disk drives supplied by International Business Machines Corporation, data fields are covered by ECC while ID fields for the data fields are not. Manufacturers of disk drives do not usually know how drives will be used or which areas on the disk will be used to store particular types of data. Only when the disk has been formatted (a process which may reoccur several times during the service life of the product) is the use to which a particular area is to be put known.
To permit formatting of disk drives by personal computer manufacturers, and by end users after disk drives have been placed in service, disk drive manufacturers subject the magnetic layers on the major surfaces of a disk drive to a so-called "surface analysis test" (SAT) to locate defects by size and area. The manufacturers then supply disk drives with a list of surface defects identifying locations and size (the "P-List"). Generally no qualification of the surface has been made on the basis of degree of defectiveness. Areas of the disk are categorized as either good or bad.